Mary Poppins She Wrote: The Life of PL Travers by Valerie Lawson 400pp, Aurum, £16.99

By about 1970 I had seen Walt Disney's Mary Poppins seven times. I know because I used to boast about it to my friends, wanting to prove that, while I might not be the grooviest little girl in the world, I was a sticker. So entranced was I with the Sherman brothers' score that I bought the sheet music for my grandmother so that she could play it on demand - and I did demand it, an awful lot. I could spell "supercalifragilisticexpialidocious" even though I had trouble - and still do - with "leopard", "business" and "potato".

Then, one day someone - a godmother, one of those childless and high-minded women who always insisted on giving me a book for my birthday instead of chocolate or a poncho - presented me with PL Travers's Mary Poppins. I sniffed it suspiciously, like a cat that has been given a dud bit of fish. This didn't look like the real deal at all. The illustrations of Mary Poppins - by Mary Sheppard - made her look like a stiff, ugly Dutch doll and nothing like lovely Julie Andrews. And the story! It was all wrong. There was no elegant Edwardian setting, but instead something pinched and shabby from the 1930s. Mary Poppins spoke in an ugly cockney accent, and didn't sing a note. Instead of just Jane and Michael there were lots of little Bankses, including spooky twins who could talk to stars.

In fact the whole Mary Poppins book was weird and off-kilter. There was endless guff about astronomy and snatches of narratives concerning dancing cows and figures trapped in porcelain that sounded a bit like something from Hans Christian Andersen but weren't as good. And who was this PL Travers, anyway, who had stolen Mary Poppins and turned her from the most important fantasy figure in my life into a snappy, common scold?

Pamela Lyndon Travers was a youngish woman when, in 1934, she wrote the first of her Mary Poppins books, books which she was emphatic were not especially meant for children. What they were, rather, were elaborate paradigms, inflected with cod-Jungianism, which spoke of the journey towards psychic wholeness undertaken by all her characters, Mary Poppins included. Much was, sensibly, thinned out from the Disney film of 1964, or else transformed into larky dance sequences featuring a tribe of cartoon penguins.

Wherever she went Travers trailed a brooding psychological fog. She had been born in Australia, the daughter of an alcoholic bank manager who dropped dead when she was seven - so tipping her into a lifelong search for a "Mr Banks" who would nurture and sustain her. (David Tomlinson's performance as a tetchy Edwardian paterfamilias doesn't come close to Travers's original creation, a wistful, seeking man who is Mary Poppins's psychic twin.) Breaking away to Britain at the first opportunity, in 1924 Travers arrived with a head stuffed full of nonsense about her father's roots in the Irish gentry (in fact he'd been born in Deptford). She proceeded to lay siege to various grand old men of the Irish literary elite, including WB Yeats, George William Russell and Francis McNamara, trying grimly to coopt them into becoming Mr Banks while bombarding them with so-so poetry.

As befits a relentless mythologiser, Travers changed her story about when Mary Poppins had arrived. She liked to give the idea that the magical nanny had appeared as a kind of visitation in 1934, wafting down on the wind and into her idyllic Sussex cottage. But the fact was that she had been trying out bits and pieces of the novel, including a short story called "Mary Poppins and the Match Man", as early as 1926. Scratch the surface of Travers's dreamy, gnomic outer self, the one that liked to talk in riddles to journalists in order to make them feel cloddish, and there was a shrewd commercial operator who knew instinctively how to run her own PR.

This is not to suggest that Travers was a fraud. But she was at the mercy of those who were. No one these days can read about the ghastly George Gurdjieff and his sidekick Piotr Ouspenksy without wondering how on earth so many sensible, educated people of the mid-20th century came under the influence of such a disreputable pair. Peddling a hodge-podge of Hinduism, the occult and some rather embarrassing dancework à la Isadora Duncan, this unlovely duo expertly fleeced their wealthy followers. Travers sat at their feet, enjoying the invitation to become, if anything, more self-absorbed than she had been before. She also relished the obligation to hop around with scarves, since her very first love had been not writing but ballet. This explains the high proportion of characters in the Mary Poppins books who break into dance, although not, sadly to my seven-year-old way of thinking, into the kind of vaudeville hoofing so expertly performed by Dick Van Dyke and the cartoon penguins.

Travers's negotiations with Disney were characteristically bad-tempered. Uncle Walt had been trying to acquire the rights to the books from as early as 1944, when he came upon his 11-year-old daughter reading them with obvious joy. But if Disney thought Travers was going to roll over with gratitude, he was sadly mistaken. Loving the attention while pretending not to, she flirted with the might of corporate America, while eventually succumbing in 1959 to a deal of $100,000 up front and a 5% cut of the gross. Her books were barely selling, her lodgers kept leaving and by getting into bed with Uncle Walt she was guaranteeing herself a steady income until the end of her very long life in 1995.

Inevitably Travers did not go easily into this new arrangement. The tiffs and walkings-out on set are legendary, as is Travers's somewhat hypocritical distancing of herself from the finished product, which won a raft of Oscars. Had she managed to live a few extra years she would, perhaps, have been grudgingly pleased with Julian Fellowes's current reworking of the Mary Poppins stories for the musical stage, which takes in far more of the darkness of her original creation.

Valerie Lawson has had the difficult job of making the highly unsympathetic Travers someone that you could bear to read about over 400 pages. That she does so is testimony not simply to her powers of empathy but to her very sure grasp of both the British and Australian contexts in which Pamela Lyndon Travers (real name: Helen Lyndon Goff) willed herself into being.

· Kathryn Hughes's The Short Life and Long Times of Mrs Beeton is published by Fourth Estate. To order Mary Poppins She Wrote for £15.99 with free UK p&p call Guardian book service on 0870 836 0875.